BUG: GroupBy.size with all-null data raises ValueError

[adamhooper]

• closes Pandas DataFrame groupby().Size() giving 'Value Error : Length of passed values is 65, index implies 0'  #23050
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[codecov]

Codecov Report

Merging #26112 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #26112      +/-   ##
==========================================
- Coverage   91.99%   91.98%   -0.01%     
==========================================
  Files         175      175              
  Lines       52384    52384              
==========================================
- Hits        48189    48185       -4     
- Misses       4195     4199       +4

Flag	Coverage Δ
#multiple	90.53% <100%> (ø)	⬆️
#single	40.73% <0%> (-0.14%)	⬇️

Impacted Files	Coverage Δ
pandas/core/groupby/ops.py	94.21% <100%> (ø)	⬆️
pandas/io/gbq.py	75% <0%> (-12.5%)	⬇️
pandas/core/frame.py	96.9% <0%> (-0.12%)	⬇️

---

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[codecov]

Codecov Report

Merging #26112 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #26112      +/-   ##
==========================================
- Coverage   91.99%   91.98%   -0.01%     
==========================================
  Files         175      175              
  Lines       52384    52384              
==========================================
- Hits        48189    48184       -5     
- Misses       4195     4200       +5

Flag	Coverage Δ
#multiple	90.53% <100%> (ø)	⬆️
#single	40.74% <0%> (-0.13%)	⬇️

Impacted Files	Coverage Δ
pandas/core/groupby/ops.py	94.21% <100%> (ø)	⬆️
pandas/io/gbq.py	75% <0%> (-12.5%)	⬇️
pandas/core/frame.py	96.9% <0%> (-0.12%)	⬇️
pandas/util/testing.py	90.61% <0%> (-0.11%)	⬇️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[jreback]

thanks @adamhooper